FIELD OF THE INVENTION
The present invention relates to nontoxic gas generating compositions that when combusted rapidly generate gases that are useful for actuating vehicle occupant restraint systems in motor vehicles and specifically, the invention relates to thermally stable nonazide gas generants having not only acceptable burn rates and sustained combustion, but also a relatively high gas volume to solid particulate ratio at acceptable flame temperatures.
The evolution from azide-based gas generants to nonazide gas generants is well-documented in the prior art. The advantages of nonazide gas generant compositions in comparison with azide gas generants have been extensively described in the patent literature, for example, U.S. Pat. Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588, 5,035,757, 5,386,775, and 5,872,329, the discussions of which are hereby incorporated by reference.
In addition to a fuel constituent, pyrotechnic nonazide gas generants contain ingredients such as oxidizers to provide the required oxygen for rapid combustion and reduce the quantity of toxic gases generated, a catalyst to promote the conversion of toxic oxides of carbon and nitrogen to innocuous gases, and a slag forming constituent to cause the solid and liquid products formed during and immediately after combustion to agglomerate into filterable clinker-like particulates. Other optional additives, such as burning rate enhancers or ballistic modifiers and ignition aids, are used to control the ignitability and combustion properties of the gas generant.
One of the disadvantages of known nonazide gas generant compositions is the amount and physical nature of the solid residues formed during combustion. The solids produced as a result of combustion must be filtered and otherwise kept away from contact with the occupants of the vehicle. It is therefore highly desirable to develop compositions that produce a minimum of solid particulates while still providing adequate quantities of a nontoxic gas to inflate the safety device at a high rate.
The use of phase stabilized ammonium nitrate is desirable because it generates abundant nontoxic gases and minimal solids upon combustion. To be useful, however, gas generants for automotive applications must be thermally stable.
Often, gas generant compositions incorporating phase stabilized or pure ammonium nitrate exhibit poor thermal stability, and produce unacceptably high levels of toxic gases, CO and NOx for example, depending on the composition of the associated additives such as plasticizers and binders. In addition, ammonium nitrate contributes to poor ignitability, lower burn rates, and performance variability. Several known gas generant compositions incorporating ammonium nitrate utilize well-known ignition aids such as BKNO.sub.3 to solve this problem. However, the addition of an ignition aid such as BKNO.sub.3 is undesirable because it is a highly sensitive and energetic compound, and furthermore, contributes to thermal instability and an increase in the amount of solids produced.
Certain gas generant compositions comprised of ammonium nitrate are thermally stable, but have burn rates less than desirable for use in gas inflators. To be useful for passenger restraint inflator applications, gas generant compositions generally require a burn rate of at least 0.4 inch/second (ips) or more at 1000 psi. Gas generants with burn rates of less than 0.40 ips at 1000 psi do not ignite reliably and often result in "no-fires" in the inflator.
Yet another problem that must be addressed is that the U.S. Department of Transportation (DOT) regulations require "cap testing" for gas generants. Because of the sensitivity to detonation of fuels often used in conjunction with ammonium nitrate, most propellants incorporating ammonium nitrate do not pass the cap test unless shaped into large disks, which in turn reduces design flexibility of the inflator.
The compositions described in U.S. Pat. No. 5,035,757 to Poole exemplify state of the art gas generant compositions that function well but produce relatively large amounts of solid combustion products. As a result, the gas produced is less than that produced by current state of the art "smokeless" gas generants. Thus, more gas generant and greater filtering requirements are required during operation of an airbag inflator.
On the other hand, compositions described in U.S. Pat. No. 5,872,329 to Burns et al. exemplify current state of the art "smokeless" gas generants. The combustion products are primarily gas with minimal formation of solids. The benefits include a reduction in the amount of gas generant required and reduced filtering requirements. However, these compositions may be disadvantaged by lower burn rates and a failure to sustain gas generant combustion. To overcome these disadvantages, a stronger and more robust inflator is often required to increase the operating pressure of the inflator and thereby improve the burn of the gas generant.
Accordingly, it would be an improvement in the art to provide the gas generant burn characteristics of compounds as described in U.S. Pat. No. 5,035,757 along with the capacity to produce more gas and less solids as exhibited by state of the art "smokeless" gas generants.